Particulate matter (PM) discharged from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbon and contains a trace of sulfate (misty sulfuric acid fraction). In order to reduce such particulate matter, conventionally a particulate matter filter has been incorporated in an exhaust pipe through which exhaust gas flows.
Such particulate matter filter has a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages. Alternate ones of the passages have plugged inlets and the remaining passages with unplugged inlets have plugged outlets. Discharged downstream is only the exhaust gas passing through thin porous walls which compartmentalize the passages.
The particulate matter in the exhaust gas, which is captured and deposited on inner surfaces of the thin porous walls, requires to be forcedly heated and burned off so as to regenerate the particulate matter filter.
Specifically, it has been conducted to arrange, in front of the particulate matter filter, a flow-through type oxidation catalyst upstream of which fuel is added so that the added fuel undergoes oxidization reaction through the oxidation catalyst and the exhaust gas elevated in temperature by resultant reaction heat is guided to the particulate matter filter to increase a catalyst bed temperature to thereby burn off particulate matter, resulting in regeneration of the particulate matter filter. As a measure for executing the fuel addition, main injection of the fuel near a compression upper dead center may be followed by post injection of the fuel at non-ignition timing after the compression upper dead center to thereby add the fuel to the exhaust gas.
This kind of particulate matter filter may have fault due to, for example, erosion of the filter through combustion of much particulate matter all at once or breakage of the filter through mechanical shock or the like. When driving is continued with the particulate matter filter having the fault, the particulate matter discharged from an engine cannot be properly removed from exhaust gas, resulting in discharge of the particulate matter as atmospheric contaminant into atmosphere.
Thus, it has been conducted to mount on a vehicle an OBD (on-board diagnostics) system which automatically determines fault of a particulate matter filter through self-diagnosis so that a driver is informed of the fault in and is urged to replace the particulate matter filter. The fault of the particulate matter filter may be determined, for example, through detection of pressures of the exhaust gas upstream and downstream of the particulate matter filter; the particulate matter filter is diagnosed as having fault when difference between the detected pressures is lower than a predetermined value.
Regulations on exhaust gas from a vehicle tend to be strengthened year by year. Especially in U.S.A., there is circumstance that installation of an OBD system for fault determination of an exhaust emission control device in a vehicle has been obliged ahead of other countries and there is a prospect in near future that the regulations on fault determination of an exhaust emission control device will be further strengthened. The above-mentioned determination through the detection of the pressures will be insufficient to cope with such severe regulations. Thus, a method has been demanded which can determine fault of a particulate matter filter more precisely than ever before.
As a device capable of determining fault of a particulate matter filter with a higher degree of precision, there has been proposed, for example, a so-called deposition-type particulate matter sensor (PM sensor) in which particulate matter is deposited on a surface of a foundation with two electrodes, voltage being applied between the electrodes to output an electric current depending on an deposition amount of the particulate matter. With such PM sensor provided downstream of the particulate matter filter, fault of the particulate matter filter can be determined on the basis of the deposition amount or a deposition velocity of the particulate matter on the PM sensor. There exists, for example, below-mentioned Patent Literature 1 pertinent to this kind of device for making fault diagnosis of a particulate matter filter. Disclosed in Patent Literature 1 is a PM sensor arranged downstream of a particulate matter filter. Assuming that the particulate matter filter is in a predetermined state, a cumulative amount of the particulate matter having passed through the particulate matter filter during a predetermined period is estimated on the basis of, for example, a driving state of an engine while the cumulative amount of the particulate matter having passed through the particulate matter filter during the predetermined period is calculated on the basis of an output value of the PM sensor. Whether the particulate matter filter has fault or not is diagnosed by comparing the calculated and estimated cumulative amounts of the particulate matter.